To reduce the oxides of nitrogen, it is known that an SCR catalyst system may be arranged in the exhaust line carrying exhaust gas in an exhaust system, where SCR stands for selective catalytic reduction. Upstream from the SCR catalyst system, an injection device may be provided on the exhaust line, so that a so-called AdBlue system (AdBlue®), i.e., an aqueous urea solution, can be injected into the exhaust line. During operation of an internal combustion engine equipped with this type of exhaust system, the injection device sprays urea solution into the exhaust stream at a predetermined rate of metered addition. By hydrolysis, urea forms ammonia, triggering conversion of the oxides of nitrogen to nitrogen and water in the SCR catalyst.
For reasons of space, it is also customary to arrange the exhaust system in a motor vehicle along an underbody. Due to the low arrangement of the individual components of the exhaust system, any condensation fluid that develops can collect in the area of these components. With the SCR catalyst systems, it has been found that bearing mats with which individual catalyst elements are supported, held, protected from vibration and thermally insulated in a housing of the SCR catalyst system are arranged in the area of the condensation fluid that is formed and thus become fully saturated with it accordingly. As a rule, if the temperature of the exhaust system is high enough, the condensation fluid evaporates without leaving a residue, so that usually there is no impairment of function. However, at low ambient temperatures, in short distance operation and in modern diesel engines, it has become increasingly rare for the exhaust system to reach the temperatures required for evaporation of the condensation fluid.
In conjunction with urea injection, there may also be condensation of the injected urea solution in certain operating states. In evaporation of the water content of this solution, granular deposits of urea and other nitrogen compounds may be formed, which then do not evaporate until reaching much higher temperatures. If such deposits become embedded in the bearing mats of ceramic catalyst elements, the properties of these bearing mats may undergo deleterious changes. For example, the vibration damping effect is reduced, as is the thermal insulation effect. The bearing mat can no longer adjust elastically to the thermal expansion, so the retaining effect of the bearing mat for the respective catalyst element is reduced and the catalyst elements can move around in the housing. Likewise, a bypass flow may develop in the area of the damaged bearing mats, bypassing the respective catalyst element between the housing and the catalyst element.
It is likewise known that fuel may be injected upstream from an oxidation catalyst to achieve heating of the catalyst and/or a downstream particulate filter.